It is known that the discharge stability of pulsed gas lasers is greatly enhanced by supplying a pre-ionizing radiation to the gaseous medium. One of the possibilities for obtaining pre-ionization is using an X-ray generator. An example thereof is described in WO8910003.
In general, the performance of an X-ray generator is based on the following principle:                a plasma is created as a positive ion source        the positive ions are accelerated towards a negatively polarized electrode and create secondary electrons upon collision with this electrode        the resulting electron beam is accelerated away from the same electrode and send to collide with a metallic plate        upon collision of the electron beam on the metallic plate, X-rays are generated.        
In a particle type of X-ray generator, one or multiple wires are positioned inside an ionization chamber filled with low pressure gas such as helium. As a high voltage pulse is applied to the wire(s), plasma will form in the ionization chamber creating a high density of positive ions. This type of positive ion source can be operated at very high repetition rate.
However, a general and commonly known performance limitation is that, when the plasma has not been ignited for a certain period of time (typically more than one 1 minute), its stability can be erratic,—so called jitter-effect—, or it may generate a lower ion density than usual. This is probably linked to insufficient amount of electrical charges necessary to maintain the plasma upon ignition.
Since it is further commonly recognized that the timing of the X-ray pre-ionizing beam generated by an X-ray generator with respect to the main laser discharge has a great influence on laser output energy and stability, unstable plasma may seriously compromise optimum performance of the main laser discharge.
In an attempt to solve the above problem, the amount of electrical charges for maintaining the plasma upon ignition can be increased by increasing the high voltage applied to the wire(s), or by applying a number of pulses with lower voltage subsequently to a first high voltage pulse. However, a disadvantage thereof is that the stress on the high voltage wires is increased and that a higher failure risk is present due to arcing in the ionization chamber.
It is therefore an object of the present invention to provide a method enabling improved stability of the plasma upon ignition, in particular after not been ignited for a certain period of time.
It is another object of the present invention to provide a method generating a stable and even improved ion density.
In addition, it is an object of the present invention to provide an ionization chamber improved stability of the plasma upon ignition, in particular after not been ignited for a certain period of time.
A further object of the present invention is to provide an improved X-ray generator.
Still another object of the present invention is to provide an improved laser apparatus.
The present invention meets the above objects by subjecting the plasma upon ignition to an amount of light.